Enhancing the thermal characteristics of magnetically stabilized fluidized beds

ABSTRACT

In a process that involves use of a fluidized bed of solid particulate material wherein fluidization is obtained by passge of a gas through the bed, and the bed is stabilized by including discrete magnetizable particles in the bed and applying a substantially uniform magnetic field to the bed, temperature gradients that occur in the bed when the process in which the fluidized bed is used involves release or absorption of heat are reduced by periodically removing the magnetic field in a cyclical fashion. The period of magnetic field removal is long enough to destabilize the fluidized bed with respect to the positioning of fluidized particles so that the particles will move about in the bed, but the period of removal is not so long as to permit the unstabilized bed to exhibit a boiling or bubbling effect. The ratio of &#34;on&#34; time to &#34;off&#34; time is generally within the range of from about 4 to 1 to about 4000 to 1.

SUMMARY OF THE INVENTION

This invention concerns improvements in the stabilization of a fluidizedbed of particulate solids that is employed in a process that involvesthe release or absorption of heat. More particularly, the inventionconcerns a process wherein a magnetically stabilized fluid bed of solidparticles is periodically destabilized to a sufficient extent to permitsome mixing of the component particles of the bed so as to reduce orremove undesirable temperature gradients that have developed in the bed,while at the same time preventing such great destabilization of the bedthat the latter exhibits the well-known "boiling" or bubbling effectthat is typical of an unstable fluidized bed operation.

BACKGROUND OF THE PRIOR ART

It is well-known that if a gas stream is caused to flow upward through abed of solid particles at a sufficient rate of flow, the particles inthe bed move freely instead of resting upon each other and the bedbehaves much as a liquid. These fluidized solid particles exhibitbuoyancy of floating objects, surface waves, and other propertiesnormally associated with liquids. High rate of mixing and heat transferis provided by such conventional beds, making them applicable to variousdrying, roasting, chemical and petroleum processes, as is also wellknown. A further advantage in the use of a fluidized bed in saidprocesses is that the continuous addition and removal of the solidswhich make up the fluidized bed provides for convenient means forremoval of fines formed by the breakdown of the solids and spentcatalyst particles when said fluidized solids are used in a catalyticmanner.

One serious disadvantage of gas-fluidized solids has been noted in theart, this being that as the velocity of the gas is increased to above aminimum value, bubbles are formed in the bed. A bubbling fluidized bedhas regions of low solid density comprising gas pockets or voids, thatare referred to as gas bubbles. The formation of bubbles leads tobypassing, slugging and channeling, which results in the loss of theintimate contact between the fluid and the solids that is expected in afluidized bed process.

Various methods have been tried in the prior art to stabilize fluidizedbeds by preventing the bubbling or "boiling" phenomenon, including theuse of corona discharges (U.S. Pat. No. 3,304,249) and applied magneticfields (U.S. Pat. Nos. 3,439,899 and 3,440,731). A more recent solutionto the problem is that provided by R. E. Rosensweig in copending U.S.application Ser. No. 610,071, filed Sept. 3, 1975, to which Belgium Pat.No. 834,384 corresponds. The present invention provides an improvementin the invention of Rosensweig when it is to be applied to processesthat involve heat transfer either by release of heat or by theabsorption of heat.

Briefly, in the Rosensweig invention there are included, in the bed ofsolid particulate material making up the fluidized bed, a plurality ofseparate, discrete magnetizable particles, and the bed is fluidized by astream of gas flowing upward through the bed in the usual manner. Thereis applied to the fluidized bed a substantially uniform magnetic fieldwhich is oriented with a substantial vertical component. The strength ofthe magnetic field and its deviation from a vertical orientation aremaintained so as to prevent formation of bubbles in the fluidized bedfor the existing gas flow rate and particulate solids makeup of the bed.This enables use of gas throughput rates that are as much as 10 to 20times as great as the flow rate of the gas at incipient fluidization inthe absence of the applied magnetic field, concomitant with the absenceof bubbles. Such a magnetically stabilized medium has the appearance ofan expanded fixed bed; there is no gross solids circulation and verylittle or no gas bypassing. A bed of the magnetically stabilized mediumshares many qualities of the normal fluidized bed; pressure drop iseffectively equal to the weight of the bed and independent of gas flowrate or of particle size; the media will flow, permitting continuoussolids throughput. Beds of the magnetically stabilized media also sharesome of the qualities of a fixed bed; countercurrent contacting can bereadily attained; gas bypassing is small or absent, making it possibleto achieve high conversions; and attrition is minimal.

Although magnetically stabilized fluid beds have a number of advantagesover both fixed beds and the conventional fluidized beds, including lowrates of particle attrition and high fluid flow rates at low pressuredrops, they do have one inherent disadvantage, in that they have a verylimited ability to permit the transfer of heat both between thefluidized bed and the walls that confine it, and within the fluidizedbed to and from objects immersed therein to remove heat from or to addheat to the fluidized mass. The limited ability of such beds to transfersuch heat is of little or no consequence in those cases where the bedsare being used in processes that do not involve a large release orabsorption of heat. In the great majority of applications of fluidizedbeds, however, chemical reactions and/or physical changes occur that areaccompanied by thermal effects, as for example in evaporation or dryingor in exothermal or endothermal reactions. Substantial increases intemperature within such fluidized beds can be undesirable for manyreasons. For example, they can cause thermal degradation of fluidspassing through the bed, changes in the selectivity of chemicalreactions taking place in the bed, and thermal degradation of theparticles in the bed, thereby shortening their useful lifetime. Also,when temperatures exceed the Curie temperature of the magnetic particlesin the bed they will lose their magnetic properties and thus preventstabilization of the bed with a magnetic field. Additionally, hotregions in the bed can cause gas expansion to the extent that the gasvelocity in those regions exceeds the maximum velocity at which magneticbed stabilization can be achieved for the strength of the magnetic fieldbeing employed.

Similarly, in those cases where heat is being absorbed rather thanreleased during the process occuring in the magnetically stabilizedfluidized bed, substantial declines in temperature in localized areascan also lead to undesirable conditions, including reduction in the rateof chemical reaction, reduction in the rate of physical change,condensation of a normally gaseous component of a fluid passing throughthe bed, and changes in selectivity of chemical reactions, whethercatalytic or non-catalytic.

In addition to the above-noted problems, the non-isothermal nature ofmagnetically stabilized fluidized beds makes difficult the prediction ofthe behavior of such beds, in respect to both physical and chemicalproperties.

The present invention provides a method to improve thermalcharacteristics of magnetically stabilized fluidized beds, and overcomesthe problem of thermal gradients in such beds, without sacrificing theadvantages of such beds.

This novel method for control of thermal characteristics of magneticallystabilized fluidized beds consists of periodically removing thestabilizing magnetic field from the fluidized bed and then reapplyingsaid field. The relative lengths of time in the "field on" and "fieldoff" modes is determined from the characteristics of the bed and thenature of the processing that is occurring in the bed. The moreexothermic or endothermic the process is, the less must be the ratio ofthe length of time the field is on to the length of time the field isoff. This ratio may range from as small as 4 to 1 to as great as 4000 to1, but the preferred range is from 8 to 1 to 400 to 1. The physicalconfiguration of the magnetically stabilized fluidized bed (i.e.,particle size and type, fluid velocity and physical properties, bed sizeand geometry and magnetic field strength, orientation and uniformity)determines the absolute period of "field off" mode of operation. Eachcase must be determined individually, using the criterion that the"field off" mode must end before the "boiling" or "bubbling" bed typicalof unstabilized fluidized bed operation becomes evident.

Operation is cyclical, i.e., "field on" and "field off" modes followeach other in regular succession. During the "field on" mode, themagnetically stabilized fluidized bed begins to develop temperaturegradients. Before these gradients become significant, the "field off"mode begins, during which the particles in the bed mix sufficiently toscramble any temperature gradients which appeared during the "field on"mode. The "field on" mode then resumes, whereupon temperature gradientsmay again begin to develop, only to again be scrambled by the "fieldoff" mode; and so forth.

Careful selection of the length and frequency of the "field on" and"field off" modes is necessary for proper operation of this method ofobtaining a magnetically stabilized fluidized bed that is essentiallyfree of undesirable thermal gradients. The length and frequency to givethe desired operation may be experimentally determined and the bed thenrun at that fixed condition of repetitive "field on" and "field off"modes. Alternatively, the "field on" mode may be maintained untilthermal sensors detect sufficient departure from desired absolutetemperature levels or acceptable temperature gradients, whereupon the"field off" mode follows for the desired period, followed by reversionto "field on" mode; and so forth. In this case, the "field off" to"field on" ratio is not determined in advance and held fixed; rather theratio is determined by the behavior of the process at any particulartime.

In general, the duration of the "field off" mode will not exceed twicethe residence time of the fluidizing gas in the bed; most preferably thetime off will be equal to about the residence time of the gas. Residencetime in most fluidized beds will be less than 20 seconds, more usually 4to 10 seconds. In the example which follows, wherein the area of the bedwas about 20 cm² and the gas flow rate was about 73.3 ml. per second,the gas residence time was about 4 seconds and the duration of the"field off" mode was 2 seconds. In general, the minimum time off shouldbe determined by the desired level of mixing of the solids and thepractical limits afforded by a control system for short times of the"field off" mode. The duration of the "field on" mode will be determinedby the level of temperature increase or decrease or of concentratesprofile desired or considered permissible in the particular process thatis involved. It is to be remembered that the key factor is the obtainingof solids mixing without bubbling in the bed.

The following example gives a practical demonstration of this novelmethod to improve thermal characteristics of magnetically stabilizedfluidized beds.

EXAMPLE

A cylindrical fluidized bed, 2-inch diameter, was filled with -80+100mesh, commercial nickel or kieselguhr catalyst (static bed height 15cm). The bed and feed gas were heated to 218° C. Feed gas compositionwas, by volume, 4.95% carbon monoxide, 20.2% hydrogen, balance nitrogen.At a feed gas flow rate of 4.4 liters/minute and an axially appliedmagnetic field of 508 gauss, the bed was fully fluidized and fullymagnetically stabilized. The field was maintained on for 30 seconds, andthen removed for 2 seconds, again followed by 30 seconds on and 2seconds off, repeatedly. After a few minutes of cyclical operation, abed height of 18.2 to 18.4 cm. was attained. Measurement of axial bedtemperatures was made at various periods of time with the results shownin Table I, which follows:

                  TABLE I                                                         ______________________________________                                        AXIAL BED TEMPERATURE MEASUREMENT                                             (Cyclical Operation)                                                                          Temperature In ° C After                                               Indicated Period of Operation                                 Distance From Bottom of Bed                                                                     13 Min   33 Min   58 Min                                    ______________________________________                                         4 cm             245      246      244                                        8 cm             246      246      250                                       12 cm             246      246      249                                       15 cm             247      247      249                                       ______________________________________                                    

Assays by gas chromatography showed that the exit gas containedsubstantial methane, no carbon monoxide and some carbon dioxide, thelatter being 0.34 mole per 100 moles of feed gas. Thus there were 100percent conversion of the feed gas, carbon monoxide, substantially tomethane, with some carbon dioxide being formed. Bed temperatures did notexceed 250° C. and axial differences did not exceed 6° C.

COMPARATIVE EXAMPLE

Operation of the fluidized bed of the above example was repeated withthe same degree of preheat, the same catalyst, and the same feed gas andfeed gas flow rate, but the magnetic field was held in the "on" modeconstantly at 508 gauss, instead of being pulsed with alternate "on andoff" modes. After several minutes of operation a bed height of 18 cm.was attained. Axial bed temperatures after 10 minutes and after 45minutes were as shown in Table II, which follows.

                  TABLE II                                                        ______________________________________                                        AXIAL BED TEMPERATURE MEASUREMENTS                                            (Steady State)                                                                                Temperature in ° C After                                               Indicated Period of Operation                                 Distance From Bottom of Bed                                                                     10 Min.     45 Min.                                         ______________________________________                                         4 cm             319         307                                              8 cm             253         246                                             12 cm             254         247                                             15 cm             259         249                                             ______________________________________                                    

Assays of the exit gas by gas chromotography showed no detectable carbonmonoxide, no detectable carbon dioxide and substantial methane.Comparison of the above results shows that in the steady state operationthere was a much less isothermal operation than in the pulsed operationof Example 1, i.e., a spread of 61° to 66° C. in axial bed temperaturesversus a maximum of 6° C. for the pulsed operation. It should be notedthat the pulsed operation of Example 1 afforded close to isothermaloperation without causing gas bypassing, which would have led to reducedconversion of carbon monoxide. Thus the pulsed operation permittedmovement of the particles in the bed to provide thermal mixing withoutcausing bubbling which would have led to gas bypassing. The more uniformbed temperature afforded by this invention is valuable in permittingbetter control of reactions occurring in fluidized beds and in providingbetter selectivity to desired products.

As disclosed in the Rosensweig invention, over which this inventionconstitutes an improvement, as discussed earlier in this specification,the widest range of stable behavior of the material in a magnetizedfluidized bed is obtained when the applied field is uniform. Thus, whena field is applied having a substantial vertical component to stabilizethe fluidized bed, the variation of the magnetic field to the mean fieldin the bed must not exceed 125% and is preferably no greater than 50%and most preferably no greater than 10%. The magnetizable solids in thebed preferably have a low coercivity, most preferably zero, and cancomprise all ferromagnetic and ferrimagnetic substances, including butnot limited to magnetic Fe₃ O₄, γ-iron oxide (Fe₂ O₃), chromium dioxide,ferrites of the form XO Fe₂ O₃, wherein X is a metal or mixture ofmetals such as Zn, Mn or Cu; ferromagnetic elements including iron,nickel, cobalt and gadolinium, and alloys of ferromagnetic elements. Thelarger the magnetization M of the particle, the higher will be thetransition velocity ti u_(t) up to which the bed may be operated withoutbubbling, when all other factors are held constant. Preferably amagnetizable particle of the medium will have magnetization of at least10 gauss.

The fluidized composition of matter may comprise substantially 100% ofthe magnetizable solid particles or it may comprise admixtures of saidmagnetizable solids with nonmagnetic materials. For example, suchmaterials as silica, alumina, metals, catalysts or coal may be admixedwith the above materials and the advantages of the instant inventionstill obtained. However, it is preferred that the volume fraction ofmagnetizable particles exceed 25%.

Preferably, the fluidized materials will range in particle size fromabout 0.001 mm to about 50 mm, more preferably from about 0.05 mm toabout 1.0 mm. Particles of greater dimensions will usually be difficultto fluidize, while smaller size particles will be difficult to containin any fluidized process.

This invention will find use in various processes that can employfluidized beds, including but not limited to catalytic cracking, fluidhydroforming, isomerization, coking, polymerization, hydrofining,alkylation, partial oxidation, chlorination, dehydrogenation,desulfurization or reduction, gasification of coal, fluid bed combustionof coal, and retorting of oil shale.

Although this invention has been exemplified by the use of afluidization chamber that is operated in the presence of a gravitationalforce field, it is evident that one could use other force fields,provided the flow of fluidizing gas is in the direction opposing theexternal force field. Thus, for example, the force field may be producedby centrifugal forces of a rotating system, or by the electrical forceon charged matter in an electrostatic field, or by the dielectrophoreticforce of electrically polarized matter in an electrostatic field havinga field gradient, or by forces caused by presence of a magnetic fieldgradient, or by Lorentz force resulting from passage of a current at anangle to a magnetic field, or by combinations of the foregoing.

What is claimed is:
 1. In a process for stably fluidizing a bedcontaining solid particulate magnetizable, fluidizable material withinan external force field, wherein said bed of fluidizable material isfluidized by a flow of gas therethrough with sufficient force to opposean external force field acting on said particulate material, and whereinsaid fluidized bed is subjected to a magnetic field having a substantialcomponent positioned in the direction of said external force field tostabilize said bed, the improvement which comprises: controlling thethermal characteristics of the magnetically stabilized fluidized bed bymonitoring the temperature in the fluidized bed in at least one selectedpoint within said bed by periodically removing and reapplying themagnetic field at a length of time and at a frequency such that thetemperature gradients at said selected point(s) is maintained within apredetermined range with the proviso that the magnetic field isreapplied before the bed visibly exhibits "boiling" or "bubbling"typical of unstabilized fluidized bed operation.
 2. The process of claim1 wherein said external force field is gravity, said gas flow throughsaid bed is in a generally upward direction, and said magnetic field hasa substantially vertical component.
 3. The process of claim 1 whereinthe ratio of the length of time that the magnetic field is applied tothe length of time that the magnetic field is not present lies withinthe range of about 4 to 1 to about 4000 to
 1. 4. The process of claim 1wherein the length of time that the magnetic field is not present doesnot exceed twice the residence time of the fluidizing gas in the bed. 5.The process of claim 1 wherein the gas is comprised of hydrogen, carbonmonoxide and nitrogen which are catalytically converted to an exit gascomprised of methane.
 6. The process of claim 1 wherein the solidparticulate magnetizable, fluidizable material is a catalyst comprisedof nickel on kieselguhr.
 7. The process of claim 1 wherein an exothermalreaction takes place in said bed and the magnetic field is periodicallyremoved whenever the temperature in said selected point(s) increases. 8.The process of claim 1 wherein an endothermal reaction takes place insaid bed and the magnetic field is periodically reapplied whenever thetemperature in said selected point(s) decreases.